Thomas M. Mayer

Chemical Vapor Deposition of Fluoroalkylsilane Monolayer Films for Adhesion Control in Microelectromechanical Systems

We have developed a new process for applying a hydrophobic, low adhesion energy coating to microelectromechanical (MEMS) devices. Monolayer films are synthesized from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS) and water vapor in a low-pressure chemical vapor deposition process at room temperature. Film thickness is self-limiting by virtue of the inability of precursors to stick to the fluorocarbon surface of the film once it has formed. We have measured film densities of {approx}3 molecules nm{sup 2} and film thickness of {approx}1 nm. Films are hydrophobic, with a water contact angle >110{sup o}. We have also incorporated an in-situ downstream microwave plasma cleaning process, which provides a clean, reproducible oxide surface prior to film deposition. Adhesion tests on coated and uncoated MEMS test structures demonstrate superior performance of the FOTS coatings. Cleaned, uncoated cantilever beam structures exhibit high adhesion energies in a high humidity environment. An adhesion energy of 100 mJ m{sup -2} is observed after exposure to >90% relative humidity. Fluoroalkylsilane coated beams exhibit negligible adhesion at low humidity and { 90% relative humidity. No obvious film degradation was observed for films exposed to >90% relative humidity at room temperature for >24 hr.

Focused ion beam milling of diamond : effects of H2O on yield, surface morphology and microstructure.

The effects of H2O vapor introduced during focused ion beam (FIB) milling of diamond(100) are examined. In particular, we determine the yield, surface morphology, and microstructural damage that results from FIB sputtering and H2O-assisted FIB milling processes. Experiments involving 20 keV Ga+ bombardment to doses ∼1018 ions/cm2 are conducted at a number of fixed ion incidence angles, θ. For each θ selected, H2O-assisted ion milling shows an increased material removal rate compared with FIB sputtering (no gas assist). The amount by which the yield is enhanced depends on the angle of incidence with the largest difference occurring at θ=75°. Experiments that vary pixel dwell time from 3 μs to 20 ms while maintaining a fixed H2O gas pressure demonstrate the additional effect of beam scan rate on yield for gas-assisted processes. Different surface morphologies develop during ion bombardment depending on the angle of ion incidence and the presence/absence of H2O. In general, a single mode of ripples having a ...

The Role of Interfacial Properties on MEMS Performance and Reliability

We have constructed a humidity-controlled chamber in which deflections of polysilicon cantilever beams are observed by interferometry, resulting in in-situ adhesion measurements within a fracture mechanics framework. From adhesion energy measurements for uncoated hydrophilic beams, we demonstrate an exponential dependence of adhesion on relative humidity (RH). We can explain this trend with a single-asperity model for capillary condensation. For coated hydrophobic beams, adhesion is independent of RH up to a threshold value which depends on the coating used. However, we have found that exposure to very high RH (greater than or equal to 90%) ambients can cause a dramatic increase in adhesion, surprisingly with a stronger effect for perfluorodecyltrichlorosilane (FDTS, C10H4F17SiCl3) than octadeycltrichlorosilane (ODTS, C18H37SiCl3). Newly developed computational mechanics to measure adhesion in the presence of an applied load allow us to explore how the adhesion increase develops. We believe that water adsorption at silanol sites at the FDTS/substrate interface, possibly exacerbated by coupling agent migration, leads to water islanding and the subsequent adhesion increase at very high RH levels.

Electric field induced surface modification of Au

We discuss the role of localized high electric fields in the modification of Au surfaces with a W probe using the Interfacial Force Microscope. Upon bringing a probe close to a Au surface, we measure both the interfacial force and the field emission current as a function of separation with a constant potential of 100 V between tip and sample. The current initially increases exponentially as the separation decreases. However, at a distance of less than {approximately} 500{angstrom} the current rises sharply as the surface begins to distort and rapidly close the gap. Retraction of the tip before contact is made reveals the formation of a mound on the surface. We propose a simple model, in which the localized high electric field under the tip assists the production of mobile Au adatoms by detachment from surface steps, and a radial field gradient causes a net flux of atoms toward the tip by surface diffusion. These processes give rise to an unstable surface deformation which, if left unchecked, results in a destructive mechanical contact. We discuss our findings with respect to earlier work using voltage pulses in the STM as a means of nanofabrication.

Selective area nucleation for metal chemical vapor deposition using focused ion beams

Localized growth of metal lines on Si wafers has been demonstrated using a focused Ga+ beam to selectively enhance the nucleation site density during a thermal chemical vapor deposition process. Iron and aluminum lines with thicknesses up to 2 μm have been formed using ion line doses between 4×1010 and 4×1012 Ga+/cm. Thus, the sensitivity of this process can be several orders of magnitude greater than ion beam induced polymerization techniques performed at room temperature. Auger analysis indicated that the total impurity concentration deep within the metal lines was roughly 15%. No Ga was detected in the deposited films.

Electron beam induced metalization of palladium acetate

Films of palladium acetate Pd(OOCH3)2 0.1 and 0.3 μm thick have been stoichiometrically altered through exposure to electron beams of 1–30 keV. The lowest required doses for alteration, 1000 and 2500 μC/cm2, were obtained using beam energies of 4 and 5 keV, respectively. These results have been related to Monte Carlo simulations of energy absorbed within a thin surface film. The minimum line widths of features produced was less than 100 nanometers with estimated Pd/C ratio of 1 and measured resistivities as low as 100 μΩ cm.

Focused ion beam sculpting curved shape cavities in crystalline and amorphous targets

This work demonstrates accurate sculpting of predetermined micron-scale, curved shapes in initially planar solids. Using a 20keV focused Ga+ ion beam, various features are sputtered including hemispheres, parabolas, and sinusoidal wave forms having dimensions from 1to30μm. Ion sculpting is accomplished by varying the dose at different points within individual scans. The doses calculated per point account for the material-specific, angle-dependent sputter yield, Y(θ), the beam current, and the ion beam spatial distribution. Several target materials are sculpted using this technique. These include semiconductors that are made amorphous or disordered by the high-energy beam and metals that remain crystalline with ion exposure. For several target materials, curved feature shapes closely match desired geometries with milled depths within 5% of intended values. Deposition of sputtered material and reflection of ions from sloped surfaces are important factors in feature depth and profile evolution. Materials tha...

Formation of complex features using electron‐beam direct‐write decomposition of palladium acetate

A focused electron beam has been used to selectively decompose thin films of palladium acetate, (Pd–Ac), resulting in the formation of ≤50 nm wide conductive Pd‐rich features. The formation of features from various film thicknesses of Pd–Ac has been investigated using a wide range of electron energies. Monte Carlo simulations of electron energy loss distributions in Pd–Ac films were calculated over a wide range of film thicknesses and electron energies. The Monte Carlo calculations were used to predict the volume over which the energy of the electron beam is deposited within the films. Thin conductive wires having profiles from high aspect ratio to almost circular cross sections produced by varying the electron beam energy were in agreement with the Monte Carlo predictions. Complex multilayer Pd–Ac features were then fabricated by performing overlapping exposures of Pd–Ac films with differing electron beam energies.

Effects of electron energy in nanometer-scale lithography

The effects of electron beam energy, pattern size, and resist thickness on the production of features in electron beam lithography has been investigated. The minimum electron beam energy required to produce features in films of palladium acetate ranging in thickness from 0.09 to 1.9 micrometers has been determined using beam energies ranging from 1 to 30 keV. For each of the film thicknesses, the minimum electron beam energy required to produce features was found to be closely related to the range of the electron relative to the film thickness. The pattern used to obtain these measurements consisted of a group of various sized squares. The dose required to correctly expose the squares so that the actual width was equal to the nominal width varied as the nominal width was changed within the pattern. This change in dose as the nominal width was varied, the dose factor, was used as a measure of the proximity effect. For a given pattern size, the dose factor was found to go through a maximum as the beam energy was raised above the minimum required energy. The width variance, the change in actual width of the squares as the dose was changed, was also found to go through a maximum as the beam energy was raised. As the size of the pattern was varied, the beam energies which resulted in the maximum of the dose factor and the width variance were found to be dependent on the range of the electrons relative to the size of the pattern.

Structure-Dependent Viscoelastic Properties of C(9)-Alkanethiol Monolayers

Quartz crystal microbalance techniques and in situ spectroscopic ellipsometry are used to probe the structure‐dependent intrinsic viscoelastic properties of self‐assembled CH3(CH2)8SH alkanethiol monolayers adsorbed from the gas phase onto Au(111)‐textured substrates. Physisorbed molecules, mixed chemisorbed‐fluid/solid phases and solid‐phase domain boundaries make sequentially dominant contributions to the measured energy dissipation in the growing monolayer. Deviations from Langmuir adsorption kinetics reveal a precursor‐mediated adsorption channel. These studies reveal the impact of structural heterogeneity in tribological studies of monolayer lubricants.